A sewage such as a waste water is conventionally treated as follows: As shown in a flow diagram of a conventional sewage treatment system of FIG. 6, a sewage is introduced into a primary sedimentation tank 15, in which a sludge comprising grit and screenings in the sewage is sedimented to primarily separate the sludge from the sewage. The sewage from which the sludge has thus been primarily separated is introduced into an aeration tank 16 together with a small amount of sludge returned from a final sedimentation tank 17 described later, in which the sewage is aerated. Then, the thus aerated sewage is introduced into a final sedimentation tank 17, in which a sludge remaining in the sewage is sedimented to separate the remaining sludge from the sewage. A supernatant liquid resulting from the separation of the remaining sludge is released. As described above, part of the sludge sedimented in the final sedimentation tank 17 is returned to the aeration tank 16 for reuse for the aeration of the sewage.
A slurry sludge sedimented in the primary sedimentation tank 15 is taken out therefrom, and a slurry sludge sedimented in the final sedimentation tank 17 is taken out therefrom. The slurry sludge taken out from each of the primary sedimentation tank 15 and the final sedimentation tank 17 is introduced into a thickening equipment 18, in which the slurry sludge is thickened. Then, the slurry sludge thus thickened in the thickening equipment 18 is introduced into a dewatering equipment 19, in which a coagulant 23 is added to the slurry sludge, and the slurry sludge is dewatered in the dewatering equipment 19. The sludge dewatered in the dewatering equipment 19 is incinerated in an incinerator 20. A supernatant liquid produced in the thickening equipment 18 and a filtrate produced in the dewatering equipment 19 are returned to an inlet side of the aeration tank 16. The sewage is thus treated.
As described above, a slurry sludge is conventionally thickened in the thickening equipment 18, and then dewatered in the dewatering equipment 19 provided separatedly from the thickening equipment 18.
As the thickening equipment 18 for thickening a slurry sludge, the following types are conventionally employed:
(a) a gravity type thickener, PA1 (b) a floatation type thickener, and PA1 (c) a centrifugal type thickener. PA1 (a) a centrifugal type dewatering unit, PA1 (b) a belt-press type dewatering unit, PA1 (c) a pressurizing type dewatering unit, and PA1 (d) a vacuum type dewatering unit.
As the dewatering equipment 19 for dewatering the thickened slurry sludge, the following types are conventionally employed:
The above-mentioned conventional thickening and dewatering of a slurry sludge have the following problems:
(1) When a gravity type thickener is used as the thickening equipment 18, a slurry sludge is sedimented by its own gravity and thickened. A long period of time is therefore required for thickening, and a vast site is required for thickening of a large amount of slurry sludge.
It is furthermore necessary to hold a slurry sludge at a standstill for long period of time for thickening the slurry sludge, during which the slurry sludge may become anaerobic and detrimental elements such as phosphorus contained in the slurry sludge may thus be easily dissolved into the supernatant liquid. Moreover, the long holding of the slurry sludge at a standstill makes it easier for the slurry sludge to be deteriorated, so that it is necessary to add a coagulant in a larger amount when dewatering the slurry sludge in the dewatering equipment 19, and a cakey sludge after dewatering tends to have a higher water content.
(2) When a floatation type thickener is used as the thickening equipment 18, a slurry sludge adheres to innumerable fine bubbles blown therein, floats up under the effect of buoyancy of the bubbles, and is thus thickened. A long period of time is therefore required for thickening, and a vast site is required for thickening of a large amount of slurry sludge, thus posing problems similar to those in the above-mentioned gravity type thickener.
(3) When a centrifugal type thickener is used as the thickening equipment 18, an electric power cost and a maintenance and management cost in huge amounts are necessary, with production of noise and vibration during operation, against which some measures should be taken.
(4) The centrifugal type, the belt-press type, the pressurizing type and the vacuum type dewatering units as the dewatering equipment 19 require an equipment cost and an operating cost in considerable amounts.
(5) Transfer of the slurry sludge thickened by means of the thickening equipment 18 to the dewatering equipment 19 requires many conduits and pumps, thus resulting in a high equipment cost.
Under such circumstances, there is a strong demand for the development of a method for economically and efficiently thickening and dewatering a slurry sludge, not requiring a vast site, in a short period of time with a low equipment cost, but such a method has not as yet been proposed.